I've had them since Monday & I've been putting a little food in every other day as the man in the shop suggested - it's a new tank, only been running slightly under 2 weeks, planted - the fish don't seem to be touching the food? (I have been doing my best to fish it all out).
I've had /a/ fish take a nibble once or twice, but that's all.

Should I be worried? & if so, what do I do next?

I don't have a water test kit but I've been doing water changes every other day, it's a 30 litre tank && the food I'm using is aquarian advanced nutrition tropical flake food.

I'm probably just being paranoid but since they're my first fish, I have nothing to compare their behaviour with.

That would probably work, but I would not go to that extreme. First question is, where did they come from? And I don't mean the store, I mean where did the store get them. As you are in the UK, they may well be tank raised. Cardinals have been tank-raised in the Czech for years, and UK gets a lot of their fish from Europe.

Also, this has to do with parameters. If you have hard basic water, wild caught cardinals will not last long; tank raised, if raised in similar water (another reason for finding out where they came from) will be less likely to have this problem.

What is the aquascape in the tank? Cardinals come from very dark waters, thick with plants or branches and/or overhanging vegetation. Wild fish will be highly stressed in a brightly-lit plantless tank, and even tank raised fish do not lose their natural instincts in this respect.

A group of five is OK, a few more if these settle in would be nice though. But get these settled first.

Cardinals are not fish for new tanks. This fish is highly sensitive to chemicals, minerals, etc.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]

n___n They've started eating. I just broke the flakes up a bit and it seems that's made it more appealing.

^ that's what the tank looked like a few days after I got it.
So they've got a fair few plants all except the sword at the front are doing well & under the bridge to hide if they get really stressed or w/e. But they've only done that when I've been doing water changes. (currently I'm using a jug since I don't have a gravel cleaner yet.. =/)
Also I took the pots out.
Could you recommend me a good floating plant to offer then a little more shelter?

My water is pretty hard but I'm trying to use rain water for water changes so it should become softer over time. =b

Glad to hear they seem to be settling in now. That is a nice aquascape, but provides little "comfort" for cardinals, so I agree floating plants would add a real benefit. Also, a branch placed at an angle maybe?

For floating plants, have a look at Ceratopteris cornuta in our profiles (click on shaded names to see the fish/plant profile, or use the second tab from the left in the blue bar at the top); if you can find it that would be perfect. Also, Brazilian Pennywort works very well floating. Wisteria also.

Cardinals have what Baensch termed a light phobia; given the option, they will always remain in shadows, and that means they will be healthier long-term. Have a read of our profile on this fish [Paracheirodon axelrodi] for more info.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]

I'll see what I can find in the way of floating plants when I next go to the shop. =)

I'm having some trouble with my sword plant. Some of its leaves, the new ones mainly I think, are much paler and the others and quite translucent..?
I'm wondering if that might be from lack of the right nutrients.. or possibly too much light? That corner of the tank gets a little sunlight on it in the morning.

I'll see what I can find in the way of floating plants when I next go to the shop. =)

I'm having some trouble with my sword plant. Some of its leaves, the new ones mainly I think, are much paler and the others and quite translucent..?
I'm wondering if that might be from lack of the right nutrients.. or possibly too much light? That corner of the tank gets a little sunlight on it in the morning.

Echinodorus (swords) are heavy feeders; pale leaves or yellowing leaves is almost always a sign of nutrient deficiency. Are you using any fertilization, and if yes, exactly which?

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]